The present invention relates to a so-called corrugated plastic pipe having a spiral ridge on an outer surface thereof, a method and an apparatus for manufacturing such plastic pipe.
Corrugated plastic pipes having a spiral ridge on an outer surface and a flat inner surface have been getting wider applications in houses and other constructions.
To integrally form such corrugated plastic pipes having a spiral outer layer and a flat inner wall, various methods are generally used as follows:
(a) First spirally winding a plastic ribbon in a softened state with its side edges overlapped and fused to form an inner wall, and then spirally winding a formed plastic ribbon having a longitudinal ridge around the inner wall, thereby providing the resulting plastic pipe with a spiral ridge-and-groove outer surface. PA1 (b) Forming a plastic ribbon having a longitudinal ridge and flat side edges and spirally winding the plastic ribbon with its side edges overlapped and fused to provide the resulting plastic pipe with a spiral outer surface and a flat inner wall simultaneously. PA1 (a) supplying a plastic ribbon in a softened state onto a plurality of rotating forming rolls arranged circularly, so that the plastic ribbon is spirally wound around the forming rolls while partially overlapping to be fused with each other: PA1 (b) spirally winding an endless forming core belt around the forming rolls in such a manner that the endless forming core belt is sandwiched between the plastic ribbon already wound around the forming rolls and the plastic ribbon newly supplied, thereby providing the resulting plastic pipe with a spiral ridge; PA1 (c) cutting the spiral ridge after the plastic ribbon is fused with each other and sufficiently cooled, thereby providing the spiral ridge with a slit extending along it to remove the endless forming core belt from it: and PA1 (d) winding a second plastic ribbon in a softened state around the spiral ridge to fuse the second plastic ribbon thereto, thereby sealing the slit. PA1 (a) a plurality of rotatable forming rolls arranged circularly; PA1 (b) first means positioned near the rear ends of the rotatable forming rolls for supplying a plastic ribbon in a softened state to the rotatable forming rolls; PA1 (c) an endless forming core belt, part of which is wound around the rotatable forming rolls in such a manner that the endless forming core belt is sandwiched between the plastic ribbon already wound around the rotatable forming rolls and the plastic ribbon newly supplied from the first means, thereby providing the resulting plastic pipe with a spiral ridge: PA1 (d) a cutter for providing the spiral ridge with a slit extending along it to remove the endless forming core belt therefrom; and PA1 (e) second means provided on the downstream of the cutter for supplying a second plastic ribbon which is to be fused to the spiral ridge to seal the slit.
In the above two methods, a flat or formed plastic ribbon is used to provide a hollow ridge spirally extending on the outer surface of a plastic pipe. On the other hand, it is hypothetically possible to use a forming core belt which is sandwiched between an upper plastic ribbon and a lower plastic ribbon, thereby providing a plastic pipe with a spiral ridge filled with the core belt. However, there is no means to remove the core belt after completion of the plastic pipe. Therefore, this method cannot provide plastic pipes with hollow spiral ridges.
Japanese Patent Laid-Open No. 51-37969 discloses a method of manufacturing a corrugated plastic pipe by using a flat plastic ribbon attached to the bottom surface of a forming core belt and flat plastic ribbon wound over the core belt. The forming core belt and the flat plastic ribbon are spirally wound around a core cylinder to form the corrugated plastic pipe. After solidification, the core belt and the inner ribbon are removed from the resulting plastic pipe, thereby providing a corrugated pipe having spiral surfaces on both outer and inner sides. This method is, however, disadvantageous in that the plastic pipe produced thereby cannot have a flat inner surface.
On the other hand, with respect to an apparatus for manufacturing a plastic pipe of such a structure, a conventional one comprises a plurality of rotatable forming rolls arranged on a hypothetical cylindrical surface, one end of each rotatable forming roll being supported by a support base like a cantilever and operably connected with a driving means. Each rotatable forming roll is slightly inclined with respect to the axis of the above hypothetical cylindrical surface. An extruder is positioned near the rotatable forming rolls, and an extruded resin ribbon is supplied to the forming rolls so that it is spirally wound around the forming rolls with its side edges overlapped. With all of the forming rolls rotating simultaneously in the same direction, the plastic ribbon is continuously conveyed toward the tip ends of the rotatable forming rolls while being formed into a tubular body with its side edges overlapped. Since the plastic ribbon is still sufficiently hot on the rotatable forming rolls, the partially overlapped plastic ribbon is completely formed into a plastic pipe which is withdrawn continuously from the tip ends of the rotatable forming rolls.
In the above conventional method and apparatus, however, it is generally difficult to form a spiral ridge on the outer surface of the plastic pipe without suffering from the deformation of the spiral ridge if the spiral ridge is hollow. This is because a plastic ribbon to be formed into the plastic pipe is in a softened state, namely in a state which enables the adjacent plastic ribbons to be fused with each other when overlapped. The use of a forming core belt can avoid such deformation of the spiral ridge, but it suffers from the disadvantage that a plastic pipe containing the forming core belt is generally heavy and not easy to bend.